This application is based upon and claims the benefit of priority from Japanese patent application No. 2006-235539, filed on Aug. 31, 2006, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a wavelength variable laser suited for miniaturization and lowering the cost.
With wide spread use of the Internet, high speed and large volume optical communication system is being developed and being put to practical use. In particular, since wavelength fluctuation (hereinafter referred to as wavelength charping) in time of modulation limits the transmission distance in a long distance and large volume communication system, efforts have been made to alleviate such limitation.
FP laser is used for short distance and small volume communications, and DFB (Distributed Feed Back) laser that oscillates at a single wavelength is used for communications at a transmission speed of a few Gb/s. Furthermore, in communications that perform transmission of a few 10 Km at a transmission speed of 10 Gb/s, modulation is not applied to the laser, laser is continuously oscillated, and modulation using an external modulator is performed.
In recent years, WDM (Wavelength Division Multiplexing) transmission technique in which a light signal having a bit rate of greater than or equal to 10 Gb/s is transmitted with one optical fiber is given attention, and introduction of such transmission technique to commercial system is being actively carried out.
However, a great number of light sources having different wavelengths are required in the WDM transmission system, and managing of such light sources is becoming a large issue. A wavelength variable laser that can oscillate in a single mode over a wide range is given attention as a measure for overcoming such problem. Normally, the wavelength variable laser is assumed to be used in combination with an external modulator when being applied to the long distance and large volume communication system. A symmetric Mach-Zehnder interferometer using LN (Lithium Niobium) crystals is being widely used for the external modulator.